Many proteins, such as certain plant and microbial toxins are potentially very powerful therapeutical agents for treating cancer and other diseases. Nucleic acids (DNA and RNA) are also potentially very versatile agents for treating viral diseases and genetic diseases. Because these macromolecules act on intracellular components to achieve their cytocidal, cytostatic, or specific regulatory effects, therapeutical efficacy depends on the ability of these molecules to get into cells.
Some macromolecules, including proteins, DNA, and RNA are known to be able to enter cells. In defined in vitro experiments, in which a protein, such as pokeweed antiviral peptide, ricin A chain, and other plant toxins are incubated with cells, a small fraction of the protein somehow passes through the cellular plasma membrane and inhibits certain synthetic pathways, leading to cell malfunction and death. Nucleic acid molecules can also enter calls under certain not-well-defined conditions. In routine transfection procedure, cells are incubated with DNA of specific genes and acquire specific genetic changes. Oncogenic DNA can enter cells, be inserted into host genome and transform cells. Anti-sense RNA enters cells and inhibits the expression of specific genes. Double-stranded DNA or mismatched double-stranded RNA can also get into cells to induce certain antiviral effects including interferon production.
It is not entirely clear how various types of macromolecules enter cells. In the case of some plants toxins, binding to the cell surface and translocation mechanisms across the membrane are involved. For example, when intact ricin, which is composed of two chains, the A and B chains, react with cells, the B chain binds to certain carbohydrate moieties on the surface of the cellular membrane and, by some unknown mechanism, facilitates the entry of the A chain into a cell. The A chain inactivates ribosomes, interferes protein synthetic pathways and cause the death of the cells. The A chain alone can penetrate cells but it does so very poorly and much higher concentrations (as compared to intact ricin molecules) are required. In the case of nucleic acid molecules, precipitation by calcium phosphate or by polyethylene glycol can enhance their entry into cells.
Although the exact mechanisms by which proteins, DNA and RNA enter cells are not well understood, it is likely that association with cellular membrane enhances entry into the cell. Further, pinocytosis has been proposed to play an important role, and it is known that the association of molecules with the cell membrane will enhance the endocytosis of the molecules by the cells.
Recently, George et al. (Proc. 1988 Miami Bio/Technology Winter Symposium, p. 33, IRL Press, Washington, D.C.) proposed a means for introducing diagnostically and therapeutically useful agents onto cellular surfaces. Exploiting the inherent ability of the carboxy terminal portion of cytochrome b.sub.5 to insert into cellular membranes, the authors suggest the coupling of this peptidic segment to other proteins to enable them to acquire this property.